1. Field of the Invention
The present invention relates to the separation of hydrocarbon and oxygenated compound impurities from lower alkylene oxides such as propylene oxide by an improved sequence of conventional and extractive distillation.
2. Description of the Prior Art
Monoepoxides such as propylene oxide are highly important chemicals useful in a great number of applications. An important commercial technology for producing the monoepoxides is via the catalytic reaction between the corresponding olefin and an organic hydroperoxide. See, for example, U.S. Pat. No. 3,351,635.
From the epoxidation reaction mixture, a crude monoepoxide product can be recovered by conventional distillation. The type and quantity of impurities associated with the crude monoepoxide will depend to a considerable extent on the organic hydroperoxide used in the reaction to form the monoepoxide
Where ethylbenzene hydroperoxide is used in the epoxidation reaction, for example, the crude monoepoxide will contain such heavy contaminants as phenol, ethyl benzene, methyl benzyl alcohol and acetophenone which are not found in monoepoxide formed by reaction of tertiary butyl hydroperoxide with the same olefin. In the latter case, tertiary butyl alcohol is present as a contaminant in the crude monoepoxide.
Certain impurities are common to crude monoepoxide prepared by both of the above reactions; these include methanol, methyl formate, propionaldehyde and acetone. However the amounts of such impurities present with the crude monoepoxide differs very greatly depending upon the reaction by which the monoepoxide is formed, and the monoepoxide purification procedure used to recover purified monoepoxide have differed very greatly.
In the case of monoepoxide prepared as a result of reaction of lower olefin with ethylbenzene hydroperoxide, the crude monoepoxide is first subjected to conventional distillation in order to remove the higher boiling components ethyl benzene, methyl benzyl alcohol and acetophenone as well as some of the oxygenated impurities before passing to extractive distillation for final purification.
In the case of monoepoxide containing none of the heavies listed above and containing much greater amounts of the oxygenated impurities is first subjected to one or more extractive distillation steps to remove oxygenated impurities before undergoing a conventional distillation step to recover the purified monoepoxide.
A disadvantage of the prior procedures used in the purification of monoepoxide from reaction of olefin with tertiary butyl hydroperoxide has been the large amounts of extractive distillation solvent and, consequently, the utilities used In addition, in the prior procedures, there tended to be formed significant amounts of polymers of the monoepoxide which necessitated an additional carbon treatment step as described in U.S. Pat. No. 4,692,535.
The present invention provides an improved sequence of conventional and extractive distillations steps for the purification of crude monoepoxide formed by reaction of olefin with tertiary butyl hydroperoxide and containing by weight at least 1500 ppm methanol, at least 100 ppm methyl formate, at least 150 ppm propionaldehyde, at least 3000 ppm acetone and up to 1000 ppm tertiary butyl alcohol.
Prior workers have provided extractive distillation techniques to accomplish the separation of impurities from monoepoxides such as propylene oxide U S. Pat. No. 3,838,020 shows a dual solvent extractive distillation process. U.S. Pat. No. 3,843,488 shows extraction distillation using a C.sub.8 to C.sub.20 hydrocarbon to purify propylene oxide U S. Pat. No. 3,909,366 shows extractive distillation purification of propylene oxide using C.sub.6 to C.sub.12 aromatic hydrocarbon. U.S. Pat. No. 4,140,588 uses water in extractive distillation purification of propylene oxide. U.S. Pat. No. 3,881,996 uses plural stage distillation to purify propylene oxide. East German Patent Specification uses aliphatic alcohols such as tertiary butanol in separating methyl formate from propylene oxide by extractive distillation. U.S. Pat. No. 5,006,206 uses tertiary butyl alcohol and water in the extractive distillation purification of propylene oxide.
It has previously been proposed to separate oxygen containing impurities from the propylene oxide by extractive distillation using lower glycols such as ethylene glycol and propylene glycol. See U.S. Pat. No. 3,578,568 which describes this procedure and which teaches use of solvent in amount to comprise 15 to 50% of the vapor space in the distillation zone. U.S. Pat. No. 5,000,825 describes a similar separation but one which uses much lower solvent concentrations whereby propylene oxide losses are reduced.
U.S. Pat. No. 3,477,919 teaches a method for purifying propylene oxide contaminated with impurities such as methyl formate which boil near propylene oxide. The methyl formate impurity is removed from the contaminated propylene oxide by reaction with an aqueous slurry of calcium hydroxide.
U.S. Pat. No. 2,622,060 teaches a process for separating propylene oxide from a crude reaction mixture by treatment with an aqueous alkali metal hydroxide solution.
U.S. Pat. No. 2,550,847 teaches a process for the purification of propylene oxide in a crude reaction mixture containing methyl formate by subjecting the mixture to strong agitation with an aqueous solution of an alkaline saponifying agent.
U.S. Pat. No. 3,350,417 teaches a process for purifying propylene oxide comprising parallel and serial stages of distillation and a caustic treatment to simultaneously aldolize acetaldehyde and saponify methyl formate. The solvent used in the reaction step is removed before subsequent caustic treatment.
U.S. Pat. No. 4,691,034 removes methyl formate from propylene oxide by contact with an aqueous calcium hydroxide slurry to which a solubilizer has been added. U.S. Pat. No. 4,691,035 removes methyl formate from propylene oxide by contact with a base such as sodium hydroxide in water and glycerol.
U S. Pat. No. 4,692,535 shows the removal of high molecular weight ethers from propylene oxide by treatment with an absorbent such as activated carbon.
Although a great deal of work has been done as above indicated with regard to lower alkylene oxide purification, there still exists considerable room for improvement in both the efficiency of the purification and in the quality of the product alkylene oxide.